1. Field of the Invention:
This invention relates to a digital printing apparatus for forming an image on a recording medium such as paper by spouting ink onto the recording medium. More particularly, this invention relates to a digital printing apparatus for forming an image on the aforementioned recording medium by causing the ink to be spouted by virtue of electrostatic force in a high electric field.
2. Description of the Prior Art:
An ink jet printer comprises a nozzle for spouting ink in the direction of a recording medium. The ink which has been converted into particles by virtue of surface tension after departure from the nozzle is advanced toward the aforementioned recording medium in accordance with an image signal. The ink jet printer of this construction, however, has a disadvantage that a great effort must be paid to the maintenance of the nozzle so as to prevent ink from clogging the nozzle.
As a new printer for directly recording an image on ordinary paper without use of any nozzle, a "magnetic fluid high-speed printer" has been proposed (Glossary of manuscripts for the First Symposium on Non-impact Printing Technique sponsored by the Society of Electrophotography of Japan on July 24 and 25, 1984, pages 113-118).
This printer comprises a multistylus formed of numerous styluses linearly arranged as regularly spaced in a row and a control electrode disposed at a prescribed distance from the multistylus and adapted to form an electrostatic field of high magnitude between itself and the aforementioned multistylus. In the printer, the recording medium is advanced between the aforementioned control electrode and the aforementioned multistylus. As the ink to be applied to the recording medium, this printer uses a liquid containing magnetic particles namely a magnetic ink. This magnetic ink is supplied to the multistylus. The multistylus is furnished with a magnet to be used for magnetizing the magnet ink. By the magnetic force of this magnet, undulating prominences of magnetic ink are formed at the leading ends of the styluses. When the electrostatic field of high magnitude is formed between selected styluses and the aforementioned control electrodes, it induces flight of the prominences of magnetic ink from the styluses. The printer operates by virtue of the phenomenon of the rise of the magnetic fluid in the magnetic field and the phenomenon of flight of the magnetic fluid in the electrostatic field of high magnitude. The terminal shapes of the prominences of magnetic ink correspond to the meniscuses in the aforementioned ink jet printer. Then the electrostatic force acts on the terminals, the magnetic ink in the leading end part of each of the prominences is forced to gather into a bead about 10 .mu.m in diameter and fly out dragging a tail behind. On the surface of the recording medium, an image conforming to image data is formed by applying an electrostatic force to each of the prominences of ink at the leading end of the multistylus in accordance with the image data and causing selected portions of the prominences of ink to fly toward the recording medium.
In the magnetic fluid high-speed printer of the construction described above, since powerful magnetic force acts on the magnetic particles in the portion of magnetic ink adjoining the multistylus, the affected magnetic particles of the magnetic ink is sedimented and condensed in the ink and consequently suffered to give rise to a highly magnetized portion of high density within the magnetic ink. As the result, the continuous supply of the magnetic ink to the multistylus is impeded. As the recording is continued for a long time, the record is produced with insufficient density. As this condition lasts, the printer eventually fails to produce desired record.
Further in the aforementioned magnetic fluid high-speed printer, the magnetic head which is provided with the multistylus and the ink injector for supplying the magnetic ink to the multistylus inevitably possesses a complicated structure. Moreover, since the magnetic ink is supplied from an ink tank to the aforementioned multistylus by means of a pump, the magnitude of prominence of the magnetic ink is varied by the vertical distance from the aforementioned multistylus to the liquid level inside the ink tank and the image formed on the recording medium lacks stability.